Inflammation is a localized protective response mounted by tissues in response to injury, infection, or tissue destruction resulting in the destruction of the infectious or injurious agent and isolation of the injured tissue. A typical inflammatory response proceeds as follows: recognition of an antigen as foreign or recognition of tissue damage, synthesis and release of soluble inflammatory mediators, recruitment of inflammatory cells to the site of infection or tissue damage, destruction and removal of the invading organism or damaged tissue, and deactivation of the system once the invading organism or damage has been resolved. In many human diseases with an inflammatory component, the normal, homeostatic mechanisms which attenuate the inflammatory responses are defective, resulting in damage and destruction of normal tissue.
Cell-cell interactions are involved in the activation of the immune response at each of the stages described above. One of the earliest detectable events in a normal inflammatory response is adhesion of leukocytes to the vascular endothelium, followed by migration of leukocytes out of the vasculature to the site of infection or injury. In general, the first inflammatory cells to appear at the site of inflammation are neutrophils, followed by monocytes and lymphocytes. Cell-cell interactions are also critical for activation of both B-lymphocytes (B cells) and T-lymphocytes (T cells) with resulting enhanced humoral and cellular immune responses, respectively.
The hallmark of the immune system is its ability to distinguish between self (host) and nonself (foreign invaders). This remarkable specificity exhibited by the immune system is mediated primarily by T cells. T cells participate in the host's defense against infection but also mediate organ damage of transplanted tissues and contribute to cell attack in graft-versus-host disease (GVHD) and some autoimmune diseases. In order to induce an antigen-specific immune response, a T cell must receive signals delivered by an antigen-presenting cell (APC). T cell-APC interactions can be divided into three stages: cellular adhesion, T cell receptor (TCR) recognition, and costimulation. At least two discrete signals are required from an APC for induction of T cell activation. The first signal is antigen-specific and is provided when the TCR interacts with an antigen in the context of a major histocompatibility complex (MHC) protein, or an MHC-related CD1 protein, expressed on the surface of an APC (“CD,” standing for “cluster of differentiation,” is a term used to denote different T cell surface molecules). The second (costimulatory) signal involves the interaction of the T cell surface antigen, CD28, with its ligand on the APC, which is a member of the B7 family of proteins.
CD28, a disulfide-linked homodimer of a 44 kilodalton polypeptide and a member of the immunoglobulin superfamily, is one of the major costimulatory signal receptors on the surface of a resting T cell for T cell activation and cytokine production (Allison, Curr. Opin. Immunol., 1994, 6, 414; Linsley and Ledbetter, Annu. Rev. Immunol., 1993, 11, 191; June et al., Immunol. Today, 1994, 15, 321). Signal transduction through CD28 acts synergistically with TCR signal transduction to augment both interleukin-2 (IL-2) production and proliferation of naive T cells. B7-1 (also known as CD80) was the first ligand identified for CD28 (Liu and Linsley, Curr. Opin. Immunol., 1992, 4, 265). B7-1 is normally expressed at low levels on APCs, however, it is upregulated following activation by cytokines or ligation of cell surface molecules such as CD40 (Lenschow et al., Proc. Natl. Acad. Sci. U.S.A., 1993, 90, 11054; Nabavi et al., Nature, 1992, 360, 266). Initial studies suggested that B7-1 was the CD28 ligand that mediated costimulation (Reiser et al., Proc. Natl. Acad. Sci. U.S.A., 1992, 89, 271; Wu et al., J. Exp. Med., 1993, 178, 1789; Harlan et al., Proc. Natl. Acad. Sci. U.S.A., 1994, 91, 3137). However, the subsequent demonstration that anti-B7-1 monoclonal antibodies (mAbs) had minimal effects on primary mixed lymphocyte reactions and that B7-1-deficient mice responded normally to antigens (Lenschow et al., Proc. Natl. Acad. Sci. U.S.A., 1993, 90, 11054; Freeman et al., Science, 1993, 262, 909) resulted in the discovery of a second ligand for the CD28 receptor, B7-2 (also known as CD86). In contrast with anti-B7-1 mAbs, anti-B7-2 mAbs are potent inhibitors of T cell proliferation and cytokine production (Wu et al., J. Exp. Med., 1993, 178, 1789; Chen et al., J. Immunol., 1994, 152, 2105; Lenschow et al., Proc. Natl. Acad. Sci. U.S.A., 1993, 90, 11054). B7:CD28 signaling may be a necessary component of other T cell costimulatory pathways, such as CD40:CD40L (CD40 ligand) signaling (Yang et al., Science, 1996, 273, 1862).
In addition to binding CD28, B7-1 and B7-2 bind the cytolytic T-lymphocyte associated protein CTLA4. CTLA4 is a protein that is structurally related to CD28 but is expressed on T cells only after activation (Linsley et al., J. Exp. Med., 1991, 174, 561). A soluble recombinant form of CTLA4, CTLA4-Ig, has been determined to be a more efficient inhibitor of the B7:CD28 interaction than monoclonal antibodies directed against CD28 or a B7 protein. In vivo treatment with CTLA4-Ig results in the inhibition of antibody formation to sheep red blood cells or soluble antigen (Linsley et al., Science, 1992, 257, 792), prolongation of cardiac allograft and pancreatic islet xenograft survival (Lin et al., J. Exp. Med., 1993, 178, 1801; Lenschow et al., 1992, Science, 257, 789; Lenschow et al., Curr. Opin. Immunol., 1991, 9, 243), and significant suppression of immune responses in GVHD (Hakim et al., J. Immun., 1995, 155, 1760). It has been proposed that CD28 and CTLA4, although both acting through common B7 receptors, serve opposing costimulatory and inhibitory functions, respectively (Allison et al., Science, 1995, 270, 932). CTLA4Ig, which binds both B7-1 and B7-2 molecules on antigen-presenting cells, has been shown to block T-cell costimulation in patients with stable psoriasis vulgaris, and to cause a 50% or greater sustained improvement in clinical disease activity in 46% of the patients to which it was administered. This result was dose-dependent. Abrams et al., J. Clin. Invest., 1999, 9, 1243-1225.